I'll probably sound like an idiot...but how hard would it be to fabricate a 15, 20, 25, or 30 round magazine for a .243/.308 type Remington 740/742/7400/750 rifle series. I've seen 10 round mags online from anywhere between 19$ and 32+. I'd like to get a 10 round mag too...but the vast majority of them on the market right now appear to be junk.

I've even considered the possibility of finding a magazine with close enough dimensions and then having the top of the stock magazine frankensteined onto it so I could keep the bolt hold-open.

I'd also like to know if anybody here could possibly make one, and how much it would cost.

Anything is possible if you have enough money.
What this would cost would depend on who's doing the work.

Several things to think about before spending the money.
1. A larger capacity magazine will almost always be unreliable due to the far stronger spring needed to operate it. The rifle action often just can't feed because of the spring tension.

2. These Remington semi-auto rifles are sporting rifles designed to fire a few shots at a time.
Unlike military rifles, they just weren't designed or engineered to shoot much more than a 5 round magazine with any speed.
The rifle just won't stand up to firing anything like 15 to 30 rounds rapid fire more than a few times, it that.
Over heating and broken parts will quickly cause failures.

Another thing to consider is the weight of the ammo loaded into a magazine. Note that the B.A.R. magazine held only twenty rounds. There is a limit as to how much weight a magazine spring can push up to function correctly. More than twenty rounds, they used gravity to supply the rounds, i.e. the Bren with the magazine on top of the gun, and the Sten with a magazine out to one side. And then there are drum magazines. But a box magazine under the gun could be problematic.

__________________Sometimes you get what you pay for, sometimes you only pay more for what you get.
Three shots are not a "group"...they are a "few".
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Dfariswheel:

You make a good point about the magazine springs, particularly since a lot of the .243 mags are "one size fits all" for .243, .257 roberts, .260 Rem, 7mm-08 and the likes which will all have different weights overall. I'd like a 15-20 rounder if I could get it to work...but after some measuring I realized it'd take quite a bit of modification to make that work without hindering the prone position. Heck, the 15-20 rounder may too.

However; though I don't intend to spray my 750 off like it was a semi-auto AK (nor would I believe this rifle could handle it) I disagree with it being too weak to handle more than 4-5 sustained rounds within a practical range of time. Hunting magazines in my state can hold 10 rounds and with the accuracy and potential from a rifle like this it wouldn't be outlandish for it to hang in there just as well as any long-range AR-15/10 Designated Marskman style. The barrel geometry of the .243 750 is the most optimal of them IMO since the barrel walls are so much thicker. The carbine .308/30-06 are good in 18.5" which could be due to the oscillation...but it's far more plausible that it's because the twist in the 22" .30's is 1 9.125" (Faster than a Mosin Nagant) and the carbines are 1-10"

Most people -seriously- underestimate the 750, and quite a few other hunting rifles for that matter. Unfortunately the 750 has an undeserved bad rap...the 740 and 742 rifles had various problems, including soft bolts and the mini-lug lockup system. The 7400 was a much better gun (and excellent if you find a good one), but Remington's notoriously mediocre quality control (mixed with being neglected and people being too cowardly to send a lemon in for fixing/replacement) made sure that even the 7400 kept a similar notoriety about it.

Luckily with the 750 they improved the gas system by not only moving it back and putting the port angle at 45 degrees...but what they fail to mention is the angle is pointed toward the front of the barrel. The gas has to basically slam into the back wall of the gas nipple/forend screw before it moves out, expands and shifts the almost stoneage simple piston (a stretch calling it that). This is why the new 750's are -so- incredibly consistent, moreso due to the "buffer" than the actual backward shift of the port itself. It's also had all the important internals nickel/teflon coated and the barrel extension thinger is cast now since the three biggest 7400 complaints were rust, burs (especially in the extender) and quality control between one rifle to the next (which they still wont improve because it cuts costs and most people will either resell the gun,or have a smith try to fix it)
It's only significant downside is that it's a tight gun. Like an EAA witness pistol they need to be broken in before you can depend on them not stovepiping occasionally. This caused some trouble in the beginning but it's died down.

Before Hornady stopped making their light mag ammunition there were a few cases of BAR's and R1's breaking various parts in the due to the fact that light mag was nothing more than a compressed blend of slow-burning powder. The Remington 750 handled it perfectly.

I contacted Remington a few times to make sure it wasn't just an idiot behind the phone/computer and asked them if I could use IMR 7828 or Retumbo powder in my .243 without damaging or severely shortening it's life. Every one of them said that the 750 will work with ANY load for ANY caliber it's chambered in so long as you don't go over SAAMI specs.

Ironically the 750's biggest advantage is how primitive and limited it's gas system is. a BAR has a gas adjuster so that they can tune it specifically for each caliber...and -especially- so they can provide WSM and Winchester Mag loadings which run much slower powders than the .30-06 and .308 models. The Remington probably wouldn't handle a .300/338 winmag...and it DEFINITELY wouldn't handle a WSM due to how efficient and violent the cartridge's detonation is. I'm surprised it handles the .35 Whelen to be quite honest...

And Dahermit you make a very good point about the springs. I already knew I'd need a significantly heavier spring...and this may be why a lot of the aftermarket 10 round mags only hold 9 reliably. Is there a place that specifically sells universal/heavier springs for magazines?

Hopefully more people will realize how good the 750 is compared to it's older siblings, but we'll find out eventually. A lot of people would also be surprised just how little difference there is between some hunting rifles and SHTF/Zombie rifles...(Rated AR, viewer discretion is advised) Heck the FNAR/SXAR is a BAR facelift with a 20 round mag yet it's a far more reliable design than a DI AR-15. The R1 shows potential to be even more durable (albeit ugly and 1.5-2 moa).

Thanks you two, you've helped me make up my mind about buying a quality 10 round mag. It's definitely worth getting 2-3 of the quality ones...and a much better idea than just having 1-2 4 rounders and a giant FAL magazine LOL.

Now I'll stop rambling before the first person to read this thread after my post doesn't have a StrokeSeizureHeadsplosion LOL.

The old Michigan Armament Co would do a military conversion on a 742 by converting it for an M14 magazine and attaching an M14 flashhider at lower cost than an M1A. But it would be a lot of work to replicate a modification that sank without a trace more than 30 years ago.

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